Despite advances in antiarrhythmic therapies, cardiac arrhythmias remain a major health problem, causing about 300,000 sudden cardiac deaths annually in the United States (Weiss J N et al., Circulation (1999) 99:2819–2826). Cardiac arrhythmias can occur when the electrical waves which stimulate the heart meander erratically through the heart muscle, creating disordered and ineffective contraction. The primary focus of literature and research has been on detecting when cardiac arrhythmias occur and reducing the occurrence of arrhythmias with medical therapies or lifestyle changes. Medical therapies include drugs which can reduce the occurrence of arrhythmias and implantable devices which can detect the onset of arrhythmias and apply electrical pulses to the heart to stop arrhythmias. However, oftentimes medical therapies are not employed until an individual first suffers and survives a sufficiently serious arrhythmia which signals the need for medical therapy. Consequently, there is a need for techniques and apparatus which can identify individuals at risk for developing cardiac arrhythmias prior to the onset of serious arrhythmia.